The dual SD cards are so that you can have redundancy if you install an OS on them. They can be mirrored (basically, a Raid 1)

from: https://dl.dell.com/topicspdf/poweredge-r740xd_owners-manual2_en-us.pdf

Internal Dual SD Module or vFlash card
The PowerEdge R740xd system supports Internal Dual SD module (IDSDM) and vFlash card. In 14th generation of PowerEdge
servers, IDSDM and vFlash card are combined into a single card module, and are available in these configurations:
● vFlash or
● IDSDM or
● vFlash and IDSDM
The IDSDM/vFlash card sits in the back of the system, in a Dell-proprietary slot. IDSDM/vFlash card supports three micro SD
cards (two cards for IDSDM and one card for vFlash). Micro SD cards capacity for IDSDM are 16/32/64 GB while for vFlash the
microSD card capacity is 16 GB.
NOTE: IDSDM and vFlash slot is not hot pluggable

#Iwork4dell

so what's the purpose of the vFLASH? is the dual IDSDM for booting from them instead of the hard drives? any other uses? thanks. 

IDSDM is used to install operating system or as a storage device. You can check below link for more details

https://www.dell.com/support/manuals/en-us/poweredge-r730-dsms/r730_ompublication/idsdm?guid=guid-0cbc7747-e59b-4278-90a7-c44c79218332&lang=en-us

vFlash is managed by iDRAC. User can create multiple partition on vFlash SD card using iDRAC and expose these partition to the host as a storage device. You can refer below link for more details on vFlash

https://www.dell.com/support/manuals/en-in/idrac9-lifecycle-controller-v5.x-series/idrac9_5.00.00.00_ug/managing-vflash-sd-card?guid=guid-8d4266b4-1bb0-41dc-be4f-e31005f745f1&lang=en-us